As the RF performance of a hand-held cellular telephone increases, the likelihood that it will be used by the driver of a moving vehicle also increases. This raises a safety concern. In response to this concern, the hands-free accessory to the cellular telephone has evolved. The basic purpose of the hands-free accessory is to affix the cellular telephone to the vehicle in an advantageous way, and thereby free the driver from distractions having to do with holding, manipulating and positioning the telephone.
Because the hands-free unit is an external accessory to the vehicle as well as to the telephone, the hands free-unit must live within certain design constraints. Among the most pressing of these constraints are physical bulk and aesthetics, both of which impose strict limits on the size of loudspeakers and the capacity of related audio components that can be included in the hands-free unit. As a result, hands-free units on the market today generally have poor audio-output characteristics, which compromise their effectiveness in the high-noise environment that is typical of a moving vehicle.
The prior art offers several devices to combat the limitations outlined above, but none is entirely satisfactory.
One device from the prior art is an automatic mute device for the vehicle's AM/FM radio. Such a device senses the presence of RF energy emanating from the transmitter of a cellular telephone that is powering-up to engage in a call and mutes the radio.
In U.S. Pat. No. 5,404,391, an automatic mute device comprising an antenna connected to a tuned circuit is used to sense voice channel activity on the cellular telephone. In response to sensing voice activity, the device provides a signal which disconnects power to the radio.
Another kind of automatic mute device, interposed in the audio path between the radio output and the vehicle's speakers, serves to interrupt the audio path and thereby mute the radio output in response to the electrical signal when that cellular telephone is actively engaged. This approach does nothing, of course, to improve the audio output quality of the cellular telephone, although it does remove an important source of interfering audio noise.
In a prior art refinement of the device just described, the automatic mute device includes its own power output audio amplifier. When the vehicle's radio is muted, the mute device also switches the vehicle's loudspeakers from the output of the radio to the output of the mute device's internal amplifier, and thereby makes the vehicle's speakers available to the audio signal produced by the cellular telephone.
This approach improves the audio output quality of the cellular telephone. Unfortunately, the installation of such a device is somewhat involved, as the vehicle's plurality of speakers must be routed through and controlled by the mute device. Also the volume of the mute device's audio output needs to be re-set as the level of external noise changes. Perhaps more importantly, the power output audio stages already present in the radio are unproductively duplicated in the mute device, thereby leading to unnecessary cost. Moreover, the DC electrical power required by the added power output audio stages is significant, which necessitates the presence and expense of a heavy-duty electrical service to the automatic mute device.